Piston rings are important to the efficient operation of air compressors and internal combustion engines. Since the reciprocating motion of a piston within a cylinder is the greatest contributor to friction in a compressor and an engine, the use of a lubricant such as oil between the piston and the cylinder is a must for efficient operation. The lubricant is normally provided from a crankcase adjacent the cylinder. To prevent the lubricant from leaking past the piston into a work chamber defined by the cylinder and piston, pistons are usually provided with piston rings. Piston rings are also provided to prevent compressed air or compressed combustion gases in the work chamber from leaking past the piston into the crankcase.
U.S. Pat. No. 2,895,772 to Chapman et al., for example, discloses a packing ring assembly for providing a seal between a piston and a cylinder. The assembly includes axially opposed sealing cups clamped between two pistons. Chapman et al., however, does not disclose means for efficiently directing lubricant scraped from the cylinder back into a crankcase.
U.S. Pat. No. 3,182,569 to Hieber discloses a seal structure for a piston in a cylinder. The structure includes two opposing flexible cup-type seals positioned in grooves of the piston, with the ends of the piston swaged to clamp the seals. Heiber, however, also does not disclose means for efficiently directing lubricant scraped from the cylinder back into a crankcase.
U.S. Pat. No. 3,608,911 to Prasse et al. discloses a piston for an internal combustion engine. The piston includes a U-shaped oil control ring seated within a groove and expanded outwardly by a circumferentially expanding spring. The piston also includes ports extending from the groove, and the oil control ring has holes, apparently for returning oil scraped by the ring from the cylinder. The ring, however, is not clamped to the piston, is made of metal and presumably includes a gap to allow the ring to be fitted onto the piston.
U.S. Pat. No. 5,231,917 to Wood discloses a wobble piston for use in a non-lubricated air compressor. The piston includes a flexible, annular seal clamped adjacent to the top of the piston. Wood, however, does not disclose means for efficiently directing lubricant scraped from the cylinder back into a crankcase.
U.S. Pat. No. 5,474,307 to DeBiasse et al. discloses a piston ring configuration for use in an internal combustion engine. The configuration includes wiper members that fit around gapped combustion rings. While the piston includes drain ports that allow oil scraped from the cylinder to flow into the piston and down into the crankcase, the wiper members are not clamped to the piston, thereby requiring a relatively complicated ring assembly to prevent compressed gases and oil from escaping between the combustion ring and the piston.
It should be understood that, while most of these patents disclose piston rings for use with internal combustion engines, there can be different requirements for the piston rings of an internal combustion engine and the piston rings of a compressor. For example, no lubricant at all should enter the cylinder in a compressor since the oil will undesirably contaminate the discharged compressed air. With an engine, in contrast, a little lubricant entering the cylinder is often desirable since it can help lubricate cylinder valves at an opposite end of the cylinder. In addition, piston rings for use with internal combustion engines must be designed to withstand the high temperatures caused by combustion. Engine piston rings, therefore, are usually made of rigid, heat resistant material and include a gap to allow the ring to be fitted onto the piston and allow for thermal expansion. These gaps, however, can allow lubricant to pass into the cylinder. Furthermore, because of their higher performance nature, engine piston rings are normally required to be manufactured to tighter tolerances and are, therefore, relatively expensive.
What is desired, therefore, is a piston assembly having a lubricant-scraping ring that allows a lubricant, such as oil, to be used between a piston of the piston assembly and a cylinder within which the piston reciprocates, yet prevents the lubricant from leaking past the piston. Preferably, the ring will be clamped to the piston to prevent lubricant from passing between the ring itself and the piston. In addition, the piston assembly will desirably include means for efficiently directing lubricant scraped from the cylinder back into a crankcase adjacent the cylinder. Preferably, the lubricant-scraping ring will have a flexible outer edge that will conform to the cylinder wall in order to maximize scraping while lowering manufacturing tolerances. Furthermore, it is desired that the outer edge of the ring be continuous, as opposed to having a gap as some piston rings do, to ensure that the lubricant does not have any gaps to pass through.